1. Technical Field
This invention pertains to the removal and attachment of surface mounted devices, such as integrated circuit chips, with respect to printed circuit boards. Surface mounted devices are usually connected to ne face or side of the printed circuit board by soldered connections. Although some chips are connected to boards by connections going through holes in the board, surface mounted devices are connected by solder attached only to the face of the substrate on which the surface mounted device lies. The soldered connections may attach a plurality of sides of the surface mounted device to the board. In fact, typical surface mounted devices are attached to a printed circuit board by soldered connections on each of the four sides of the rectangular surface mounted device. On the other hand, many of the leadless surface mounted devices are attached by soldered connections under the device, rather than on its sides.
Numerous situations may result in a need for removing a surface mounted device which has been soldered to a printed circuit board. The chip may have been erroneously attached in an incorrect position on the substrate. In other cases, the surface mounted device may cease to function, so that a new chip must be substituted for the old one.
1. Background Art
Much of modern industry is dependent on properly functioning printed circuit boards. When a board is incorrectly assembled, or when a chip needs to be replaced, soldered connections must be cleanly broken to allow the incorrectly placed or disfunctioning chip to be removed. A tool for removing malfunctioning chips would be most useful if it was portable and compact enough to be used at the location of equipment containing the malfunctioning chip.
Soldered connections may be individually desoldered by means of a tool, such as a soldering iron, which is placed in contact with the connection to apply heat. Heat may be so applied to raise the soldered connection to its melting point, in the case of eutectic solder. Similarly, the temperature of non-eutectic solder may be raised until the solder assumes a plastic phase. In either case, the connection may harden anew when the solder between the chip and the board is allowed to return to room temperature. Thus, a chip attached to a board by soldered connections may be removed only if all of the soldered connections holding it in place are heated simultaneously to the melting point or plastic phase as the case may be.
A tool designed to desolder by applying heat to individual connections is described in U.S. Pat. No. 3,172,382 to Weglin. Heat is conductively applied to each connection, and a hand manipulated bulb is used to remove loosened solder by applying suction through a metal tube. Although this tool effectively desolders single soldered connections, it is not an effective device for desoldering surface mounted devices, which are held in place by multiple soldered connections, all of which must be broken at once.
Complicated devices have been designed to direct heat to all sides of a surface mounted device, so as to heat each connection at the same time. One such device is shown in U.S. Pat. No. 4,366,925 to Fanene. A hot gas source is used to direct heat onto the component to be removed, while a needle inserted at the base of the component is used to force the component upwards. The device is bulky and complicated. Adjacent surface mounted parts may interfere with placement of the needle, and the needle may injure the surface mounted device.
Similarly, U.S. Pat. No. 4,552,300 to Zovko, et al. describes a method and apparatus for applying heat to a chip, which is then removed by means of a vacuum device. The mechanism is so large and bulky that the suction head, when lowered into contact with the chip to be removed may mechanically interfere with adjacent chips on a circuit board containing multiple densely mounted components.
The Fanene and Zovko patents share additional problems. As hot gas is used to increase the temperature of the soldered connections on the chip to be removed, that hot gas may flow to adjacent chips on the substrate, possibly causing connections on these near-by chips to become desoldered.
Various mechanisms have been utilized to avoid the detrimental effects of hot air flowing to soldered connections of chips adjacent to the surface mounted device being removed. U.S. Pat. No. 4,626,205 to Barkley et al. discusses an apparatus which directs a heated fluid to the connections of a chip to increase temperature. This heated fluid ma be directed through a nozzle structure to the edges of the surface mounted device being removed, without increasing the temperature of adjacent soldered connections. However, chips positioned close to the surface mounted device to be removed may mechanically prevent the fluid containing nozzle from being lowered onto that surface mounted device. Additionally, the bulk of this machine makes it impossible to transport it from its dedicated location.
Similarly, a heat source is applied conductively to the soldered connections of a chip to be removed in the devices described in U.S. Pat. No. 4,528,746 to Yoshimura and in U.S. Pat. No. 4,066,204 to Wirbser et al. Such a conductive heat source has the disadvantage that it is impossible to determine the exact moment in which the soldered connections are sufficiently heated to reach the melting point or plastic phase, thus resulting in the continued application o heat in excess of that needed to remove the chip. Such excessive heat may adversely affect the surface mounted device, and the substrate to which it is connected. Although the Wirbser device heats the entire chip, and thus may eventually sufficiently increase the temperature of solder under the chip to facilitate removal of leadless surface mounted devices, this lengthy heating of the chip itself may adversely affect the chip, and the board on which it is mounted.
Other methods of overcoming the problem of allowing hot air to leak to adjacent components have been attempted. U.S. Pat. No. 4,605,152 to Fridman discusses a device for delivering heated air to a surface mounted device, which is removed by means of a vacuum. The hot air is directed to the chip to be removed through a nozzle structure. Similarly, U.S. Pat. No. 4,426,571 to Beck describes an electric hot air tool utilizing differing size nozzles to direct the hot air onto the chip to be desoldered. The size and complexity of these apparatuses are detrimental. A circuit board from which a chip is to be removed must be taken to the dedicated location in which the apparatus resides. Taking the Fridman or Beck device to the location of the circuit board is virtually prohibited by their size and cost.
U.S. Pat. No. 4,564,135 to Barresi et al. describes a nozzle-type structure for heating the soldered connections of a surface mounted device, while inhibiting the directed hot air from heating either adjacent chips or the surface mounted device being removed. The interior design of the Barresi nozzle does surpass the nozzle designs described in Fridman and Beck, by preventing hot air from increasing the temperature of the chip being removed. The device may still result in needless and detrimental heat being applied to the removed chip, however. Since there is no way to know when the soldered connections have been completely melted, the user of the Barresi apparatus is likely to apply heat to the chip being removed for a longer period of time than is necessary for removal, thus causing damage to the substrate to which it is attached. The chip is not automatically removed from the board when the soldered connection reaches a molten state.
Other chip desoldering devices known in the prior art are described in U.S. Pat. No. 4,569,473 to Guiliano and U.S. Pat. No. 3,557,430 to Jones. These devices effectively remove chips which are connected to the printed circuit board by soldered connections that go through holes to the underside of the board, rather than residing on the surface. Although these inventions are effective for their intended purpose, they do not establish a device or method of removing surface mounted devices.
The devices described in the Zovko, Fridman, Barresi, Barkley and Bech patents may also be utilized to apply heat to a surface mounted device for the purpose of heating solder on all sides of the chip to form soldered connections with the substrate adjacent to the component. The bulkiness of these devices, and in some cases the flow of hot air onto adjacent components, results in the same adverse consequences as the use of these devices for desoldering.
Although devices are known in the prior art which effectively remove surface mounted devices from the printed circuit boards to which they are attached and in some cases facilitate soldering a surface mounted device to a substrate, these devices share certain detrimental aspects. The size and bulkiness of most of the known apparatuses for removing surface mounted devices have two draw-backs: (1) the apparatuses must be kept in a dedicated location, and are not easily transported to the location of the surface mounted device to be removed, and (2) the printed circuit board and chips adjacent to the surface mounted device to be removed are subjected to adverse heat or physical interference. Furthermore, the surface mounted device being removed or attached is disadvantageously affected by the application of extraneous heat, since the surface mounted device is not shielded from the application of heat, or is not removed from the board and the heat applier until some time after the soldered connections have been melted or secured.